Liquid crystal displays (i.e., LCDs), which are one of conventional typical thin panels, are widely used in devices such as monitors for personal computers or mobile data terminal devices by taking advantages of their merits such as low power consumption, small size, and light weight. They also have been widely used in TV applications in recent years.
In general, the display mode of liquid crystal displays is roughly divided into a longitudinal electric field mode typified by a twisted nematic (TN) mode, and a transverse electric field mode typified by an in-plane switching (IPS) mode and a fringe field switching (FFS) mode.
Liquid crystal displays employing the transverse electric field mode feature wide viewing angles and high contrast. Liquid crystal displays employing the IPS mode execute displays upon application of a transverse electric field to liquid crystals, which are sandwiched between opposite substrates, but they cannot adequately drive liquid crystal molecules located immediately above pixel electrodes because the pixel electrodes and common electrodes, to which a transverse electric field is applied, are formed in the same layer. Thus, the liquid crystal displays have low transmittance.
In liquid crystal displays employing the FFS mode, on the other hand, an oblique electric field (fringing field) occurs because common electrodes and pixel electrodes are disposed with an interlayer insulation film sandwiched in between. It is thus possible to sufficiently apply a transverse electric field to even liquid crystal molecules located immediately above the pixel electrodes and to adequately drive these liquid crystal molecules. As a result, the liquid crystal displays can exhibit higher transmittance than the IPS mode liquid crystal displays, as well as having wide viewing angles.
Besides, the liquid crystal molecules in the liquid crystal displays employing the FFS mode are driven by a fringing field generated between slit electrodes provided in an upper layer for control of liquid crystals and the pixel electrodes disposed via an interlayer insulation film in a layer below the slit electrodes. With this configuration, a decrease in the aperture ratio of pixels can be suppressed by forming the pixel electrodes and the slit electrodes of an oxide-based transparent conductive film such as indium tin oxide (ITO) containing indium oxide and tin oxide, or InZnO containing indium oxide and zinc oxide.
Moreover, unlike the liquid crystal displays employing the TN mode, the liquid crystal displays employing the FFS mode are not always required to form separate patterns of storage capacitors in pixels because the pixel electrodes and the slit electrodes form storage capacitors. Thus, the aperture ratio of the pixels can be kept high.
Conventionally, switching devices of thin-film transistors (i.e., TFTs) for liquid crystal displays have generally used amorphous silicon (a-Si) as a channel layer of a semiconductor. One chief reason for this is that amorphous properties allow a film to be formed with highly uniform characteristics even on a large-area substrate. Another chief reason for using amorphous silicon is its high compatibility with liquid crystal displays for general TVs because amorphous silicon can be deposited at relatively low temperatures and therefore can form a film even on low-cost glass substrates, which are inferior in heat resistance.
However, thin-film transistors using an oxide semiconductor as a channel layer have been developed actively in recent years. The use of an oxide semiconductor makes it possible to stably form a highly uniform amorphous film by optimizing the composition of the film. Such a film using an oxide semiconductor has higher carrier mobility than conventional films using amorphous silicon, and therefore, has an advantage of capable of manufacturing a compact, high-performance thin-film transistor. Accordingly, a liquid crystal display with a high pixel aperture ratio can be achieved by applying such an oxide semiconductor film to a thin-film transistor on an array substrate of liquid crystal pixels.
Additionally, low carrier mobility of amorphous silicon causes the need to install a separate driving circuit for applying a driving voltage to thin-film transistors of liquid crystal pixels. In contrast, if a thin-film transistor using an oxide semiconductor with high carrier mobility is used as a driving circuit, it is possible to prepare a driving circuit on an array substrate of liquid crystal pixels. This eliminates the need to install a separate driving circuit, thereby making it possible to prepare a liquid crystal display at low cost and to narrow a frame area of the liquid crystal display, the area being necessary to install a driving circuit.
In the case of amorphous silicon, the predominant structure is a back channel etching (i.e., BCE) structure in which a channel region of a semiconductor layer is subjected to wet etching during formation of source and drain electrodes. However, if an oxide semiconductor is applied to such a thin-film transistor having the back channel etching structure, a channel cannot be formed because the oxide semiconductor will also be etched during wet etching of the source and drain electrodes.
In order to overcome this problem, for example, Patent Document 1 discloses a structure in which a channel protective film of silicon (Si) is formed on a channel of an oxide semiconductor. With this structure, it is possible to form the channel of the oxide semiconductor because the oxide semiconductor is not subjected to wet etching during wet etching of the source and drain electrodes, which is conducted after formation of the channel protective film. It is thus possible to prepare a thin-film transistor using an oxide semiconductor as a channel layer.
Note that, for example, Non-Patent Document 1 implies that electrons are stored in a region of a channel protective film that overlaps with a drain electrode.